The present invention relates to a method of separating and purifying a spent solvent discharged from a solvent extraction process in a nuclear fuel cycle, such as a reprocessing plant of spent nuclear fuel or a nuclear fuel manufacturing plant.
The present invention can preferably be utilized in regeneration and disposal processes of such a spent solvent as described above.
A solvent prepared by diluting a phosphate, such as tributyl phosphate (TBP), with a higher hydrocarbon, such as n-dodecane (hereinafter referred to simply as "dodecane") and kerosine, is widely used in a solvent extraction step of a reprocessing process of spent nuclear fuel or of a wet scrap recovery process in a nuclear fuel manufacturing plant.
The spent solvent generated in the solvent extraction step contains deterioration products, such as dibutyl phosphate (DBP), formed as a result of degradation of a portion of TBP by an acid, heat, radioactive rays, etc. Such deterioration products adversely affect the extraction when the spent solvent is recycled for reuse. Therefore, the deterioration products are removed by alkali washing with an aqueous solution of sodium hydroxide or sodium carbonate. A radioactive waste containing the deterioration products thus removed, such as DBP, is converted into a vitrified solid or a bituminized solid by mixing the same with a vitrification additive or a bituminization additive. However, in order to stabilize large amounts of the sodium component incorporated by the alkali washing, it is necessary to use a large amount of these additives. Consequently, the development of a method of separating and recovering a spent solvent which enables deterioration products, such as DBP, to be removed without using sodium has been desired in the art.
On the other hand, methods such as vacuum freeze-drying and low-temperature vacuum distillation wherein the boiling point difference is utilized have been used as a method of separating TBP, DBP and dodecane from a spent solvent. However, they are disadvantageous in that the treatment capacity is small. Consequently, the development of a separation method having a large treatment capacity for a spent solvent has been desired in the art.
Moreover, when a spent solvent is heated to conduct distillation into components, there occur problems involving the danger of fire and also the danger that volatile nuclides undergo evaporation and sublimation upon heating, thus causing environmental contamination.
In order to eliminate the above-described problems, a proposal has been made on a method of separating and purifying a spent solvent, which comprises treating the spent solvent at a temperature not greater than the freezing point of the higher hydrocarbon but not less than the freezing point of the phosphate to separate the spent solvent into a frozen solid mainly composed of the higher hydrocarbon and a remaining solution containing the phosphate in a higher concentration (see Japanese Patent Application No. 95351/1990). This solvent solidification method, however, requires a high energy due to the necessity of a low temperature not above -9.6.degree. C. which is the freezing point of the dodecane or not below -80.degree. C. which is the freezing point of TBP, so that the treatment capacity cannot be increased to a large extent.